Synchronous electrical machines include synchronous electric motors and synchronous electric generators. A synchronous electric motor is an AC (alternating current) motor in which, at steady state, the rotation of the rotor is synchronized with the frequency of the supply current. In other words, the rotation period of the rotor at steady state may be equal to an integral number of AC cycles of the supply current. A synchronous generator may provide one or more synchronous motors with electrical power to spin the synchronous electric motors and any mechanical loads on the synchronous motors. Synchronous electrical machines produce (for example, motors) or consume (for example, generators) torque when the mechanical speed of their rotors multiplied by the number of machine pole pairs is equal to the electrical speed (frequency of the generated or supplied current) and the rotor and stator magnetic poles are misaligned.
Synchronous motors are not generally self-starting motors. Because a synchronous motor produces almost no torque when the rotor is not moving, the synchronous motor cannot accelerate to a substantially higher fixed supply current frequency without a supplemental mechanism. Synchronous motors commonly cannot self-start because connecting the nonrotating motor to the electrical grid at precisely aligned stator and rotor magnetic poles is impractical and because developing sufficient torque to accelerate the rotor to the fixed frequency of the electrical grid before the misalignment exceeds ninety degrees is impractical. Misalignment exceeding ninety degrees may cause the synchronous motor to stall. Indeed, accelerating torque decreases and becomes negative when the misalignment exceeds one hundred eighty degrees.